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Hi Osim-community,

I'm working with the Arnold-Model, resp. a modified version of it.
During my simulations, I get always warnings looking like this:

--------------------------------------------------------------------------------------------------
WARNING: Thelen2003Muscle::computeInitialFiberEquilibrium(SimTK::State& s)
Continuing with an initial fiber force and length of 0 and 0.037818
Here is a report from the routine:

Solution Error : 2.696505 > tol (0.000009)
Newton Iterations : 200 of max. iterations (200)
Check that the initial activation is valid, and that the whole
length doesn't produce a pennation angle of 90 degrees, nor a fiber
length less than 0:
Activation : 0.050000
Whole muscle length : 1.#QNAN0
--------------------------------------------------------------------------------------------------

Normally the simulations finish, but sometimes they even keep sticking at a certain time step.
I assume it has to do with the large knee-angles (0-100°) during the stair-walking but have no clue what parameter to tweak in order to correct the model. I tried to individually adjust every muscle parameter but always get the same errors at the same time-steps. Has someone any experience in tracking down such errors? What parameter to change in order to find the cause for that warnings?
Thanks in advance!

The Thelen2003Muscle couldn't find a solution (exceeded the maximum number of optimizer iterations) to the computeInitialFiberEquilibrium method.

This is most likely occurring with muscles at your large knee-angles. The models were not designed or tested for large flexion angles. Changing the parameters is probably not going to help, you would have to change the force-length curves. This is difficult for Thelen type muscle models. Some people switch to the Millard type muscle model to customize muscle functions.